A mother-to-be can one day learn about her unborn baby’s risk for genetic diseases without a large needle piercing her womb.

Using just a tiny sample of the mother’s blood, this new technique can scan the fetus’s entire genome – picking up indications of congenital diseases like cystic fibrosis and sickle-cell anemia and pretty much everything there is to know about the baby’s genetic make-up.

Traditionally, prenatal testing for genetic diseases requires amniocentesis -- taking a fetal tissue sample, either from the placenta or fluid in the womb, which can harm the fetus, however slightly (although, 1% of these procedures result in miscarriage).

Over a decade ago, chemical pathologist Y. M. Dennis Lo from the Chinese University of Hong Kong discovered fetal DNA floating in pregnant women's plasma – the yellowish liquid where blood cells are suspended. That discovery has since led to noninvasive, but limited, prenatal screening for certain diseases like Down syndrome and specific genetic characteristics, such as blood type.

Now, Lo and colleagues show that the next generation’s entire genome is concealed in those fetal DNA bits floating around (cell-free) in its mother’s blood.

But, the baby’s floating DNA is mixed in with the mom’s – making up only 10% of the total mix. On top of that, all the DNA molecules in the plasma are short broken fragments. “Imagine millions of jigsaw puzzle pieces and then put in 10 times as many pieces from another jigsaw and you try to assemble the first one,” Lo says.

As verification, the researchers recruited a couple that wanted prenatal testing for an inherited and possibly fatal type of anemia called β-thalassemia. Both parents carry the mutation.

Using blood from the pregnant woman, the team sequenced billions of DNA molecules, and navigated their way using genetic maps of the mother and father. The fetus, they revealed, had inherited the mutation for the blood disease from the father, but without a faulty one from the mother, the child will only be a carrier for the disease.

At the moment, the three-week-long procedure would cost about $200,000 per case. "Cutting costs will be very important," says Lo. While sampling the entire fetal genome may remain prohibitively expensive for some while, Lo hopes within a year to develop a test focused on about five important genetic conditions.

“For any particular family you want to screen, there are only a handful of genes that are clinically important,” Lo says. These targeted tests focus in on disease-causing genes and could be 50 to 100 times cheaper, he adds.

“We're about a factor of 100 away from commercial capability, but that will change," says co-author Charles Cantor, chief scientific adviser at the genetic testing firm Sequenom, based in San Diego.